Xenon Compounds

For a long time, scientists believed that you could not make covalent compounds using Noble Gas, Group 18, elements because they are just not reactive. All these elements have a full valence shell of electrons (s²p⁶ for all the Noble Gases except helium, which has no p electrons) so there is no tendency to gain or lose electrons.

In 1933, Linus Pauling used his new concept of electronegativity, the power of an atom to attract electrons, to predict the possibility of making compounds using the heavier Noble Gas elements and fluorine or oxygen.

Fluorine, with an an electronegativity of 4.0 is the most electronegative element in the Periodic Table, so atoms of fluorine have the greatest power to attract electrons. Oxygen is the second most electrongetative element with an electronegativity of 3.5. So, if you want to try to make a compound with an "unreactive" non-metal like a Noble Gas, you should probably use a reactive non-metallic element like fluorine or oxygen.

Now, as you go down any Group of the Periodic Table from top to bottom, the electrons in the highest energy levels (the valence electrons) are less strongly attracted to the nucleus because they are further away from the nucleus. So, if  you want your "unreactive" Noble Gas atom to share electrons with your more reactive fluorine or oxygen atom, you will want to use a Noble Gas from the bottom of the Group, that is, xenon or radon.

So why didn't Linus Pauling predict radon compounds instead of xenon compounds then?

Radon is a radioactive element which occurs naturally as the nuclear decay product of uranium or thorium.

The real problem, as far as the production of radon compounds is concerned, is that its most stable isotope, radon-222, has a half-life of just 3.8 days, not much time for the preparation, isolation, and structural determination of your new compounds!

Xenon, on the other hand, occurs in the Earth's atmosphere in trace amounts, about 0.09 ppm (by volume) and naturally occurring xenon includes 8 stable isotopes.

In June 1962, Neil Bartlett reported that xenon reacted with platinum hexafluoride to produce xenon compounds ( a mixture of XeFPtF₆ and XeFPt₂F₁₁).

In September 1962, Howard Claasen reported the production of XeF₄ by reacting xenon and fluorine at high temperature.

XeF₂ can actually be produced simply by exposing xenon and fluorine gases to sunlight ! 

Many xenon compounds have now been synthesized, and many of these compounds include the electronegative elements fluorine and/or oxygen.

Further Reading

 Periodic Table

Electron Configuration

Trends in Electron Configuration

Trends in Electronegativity

Nuclear Decay

Parts Per Million Concentration

Isotopes

Ligands and Complex Ions

Suggested Study Questions

1. Write the  electron configuration for each of the following elements:
• helium
• neon
• argon
• fluorine
• oxygen
2.  Which of the Noble Gas, Group 18, elements is expected to have
• the largest atomic number
• the largest number of electrons in an atom
• the largest atomic radius
• the lowest first ionization energy
• the lowest electronegativity
3. Uranium-234 undergoes alpha decay to produce thorium-232. Write a nuclear decay equation for this reaction.
4. Thorium-232 undergoes alpha decay to produce radium-228. Write a nuclear decay equation for this reaction.
5. Radium-228 undergoes spontaneous alpha decay to produce radon-224. Write a nuclear decay equation for this reaction.
6. If you are given 25 grams of radon-222 today, how much radon-222 will remain in 38 days time?
7. Radon-222 emits an alpha particle to produce polonium-218. If you were give 10 grams of radon-222 19 days ago, what mass of polonium-218 has been produced since then?
8. Write the formula for platinum hexafluoride.
9. Give the systematic names for
• XeF₂
• XeF₄
• XeF₆
10. Explain why no compounds of neon have so far been produced.

Bismuth-209

Until recently, bismuth-209 was considered to be a stable isotope, in fact, it was thought to be the heaviest stable isotope, but scientists weren't very happy about the "stable" designation. Measurements of atomic mass and nuclear decay schemes since the 1940's indicated that bismuth-209 should be just a bit too heavy to be truly stable. Then, during the night of March 14-15 2002, a team of scientists from the Institut d’Astrophysique Spatiale d’Orsay (IAS - CNRS, Université Paris XI) stumbled across the alpha decay products of bismuth-209. 

The half-life for the decay of bismuth-209 to  thallium-205 has been reported as 1.9 x 10¹⁹ years. 
In comparison, the estimated age of the universe, starting  from the time of the Big Bang, is just  13.75 x 10⁹ years. Bismuth-209 is therefore a primordial isotope, one that has existed in its current form since before the Earth was formed. 288 primordial isotopes are known, including the 255 stable isotopes plus 33 unstable isotopes with exceptionally long half-lives like bismuth-209.

Reference

http://www.cnrs.fr/cw/en/pres/compress/bismuth.htm 

Further Reading 

Isotopes
Properties & Uses of Radiation
Nuclear Decay
Half-life Calculations

Suggested Study Questions

1. For an atom of bismuth-209, give the
• symbol
• atomic number
• mass number
• number of protons in the nucleus
• number of neutrons in the nucleus
2. For bismuth-209,
• Calculate the ratio of neutrons to protons
• If the nucleus of a heavy metal atom is considered to be stable of the neutron to proton ratio is 1.5:1, would bismuth-209 be predicted to be stable or unstable? Explain your answer.
3. For an atom of thallium-205, give the
• symbol
• atomic number
• mass number
• number of protons in the nucleus
• number of neutrons in the nucleus
4. Give the symbol for an alpha particle.
5. Write an equation for the nuclear decay of bismuth-209 to thalium-205.
6. Current predictions are that the Earth will be demolished during the death of the Sun in about 7 x 10⁹ years. Assuming you had 1 tonne of bismuth-209 today,
   • How much bismuth-209 would remain in 7 x 10⁹ years?
   • How much thallium-205 would have been produced as a result of bismuth-209 decay?
7. Bismuth-209 is produced on earth when lead-209 undergoes beta decay. Write an equation to represent this nuclear reaction.
8. The half-life of lead-209 is about 3.25 hours. If you had isolated 60 grams of lead-209, how long would it take for you to have less than about 1 gram left?

Exam Technique 1

I've never written a blog post about exam technique before, but after reading the following question on an exam paper, I felt the time had come to broach the subject.

Imagine being a High School Chemistry student who is confronted by the following scenario on an exam paper:

• Written information that tells you that the diagram refers to an experiment to model a process and observe the colour change of crystals.
  
• A diagram of a dessicator (labelled as a sealed container) containing a watch glass of copper(II) sulphate pentahydrate (no indication of colour) and a beaker labelled Liquid X.
• And the instruction that the student must identify Liquid X and explain the colour change of the crystals.

So, where do you start, given that you have so very little information to go on?

You might start with the dessicator (assuming you have recognized it as a dessicator and that you know what it is used for). This would suggest that, whatever this 'process' is, it is effected by water.

That copper(II) sulphate pentahydrate could also be a clue. It is copper(II) sulfate that is hydrated, CuSO₄.5H₂O, so if you are taking the trouble to keep moisture out of the dessicator, then maybe Liquid X in the beaker is a dessicant, something that will remove water from the air, and hence from the copper(II) sulphate pentahydrate.

Now, you would need to know, or infer from the question, that there is a colour change involved. Copper(II) sulphate pentahydrate is actually blue, while anhydrous copper(II) sulphate is off-white in colour.

But you still haven't answered the first part of the question, you must identify Liquid X. And this is where it becomes very, very, difficult. If you are sensible at this point, you might just call Liquid X a dessicant, and hope the exam markers will accept that. Otherwise you are going to come up against the nasty problem of naming a dessicant that is a liquid (not a solution, not a gel, but a liquid). So, is there such a dessicant?

Silica gel is a commonly used dessicant, but it is not a liquid, it is a very solid-like gel.

Lots of solids are commonly used as dessicants, including activated carbon, calcium sulphate, calcium chloride, and swelling-clay minerals like bentonite. You can even use sodium hydroxide as a dessicant.

Strangely enough, the term "liquid dessicant" is used in the air-conditioning industry, but the dessicant is not a liquid, it is usually solid lithium chloride.

Well, I haven't had any luck coming up with the name of a liquid dessicant.

Clearly, we have started off in the wrong direction. The resemblance of the vessel to a dessicator is probably a red-herring, something put into the question in order to get you to think along the wrong lines. Is the use of hydrated copper(II) sulphate also a red-herring?

Well, I'll keep working on the problem, but, if I was a student trying to complete the exam within the time-limit, I would have realized that this question was only worth 3 marks, and I couldn't afford to spend more than about 5 minutes on it....